Regex Find Pattern of 4 Characters

I'm confused how to do this.
I have an array of strings that match this pattern:

AABC,
ABBC,
ABCC,
AAAB,
ABBB

I'm hoping to use regex to figure out if a particular string matches one of these patterns. The strings themselves do not actually consist of A, B, or C. This is just to show how the pattern looks like. For example:
Pattern AAAB should not equal BAAA.